The invention relates to data transmissions on a bus. More particularly, the invention relates to enhancing the quality of data transmissions between differential devices and single-ended devices on a bus.
One common computer bus is called a Small Computer System Interface ("SCSI") bus. The SCSI bus can be used for coupling physically small computers with peripheral devices, including disk drives, tape drives, printers, compact disk read-only memories ("CD-ROMs"), and scanners, among others. SCSI compliant devices conform to standards maintained by the American National Standards Institute, New York, N.Y., U.S.A., designated ANSI X3T9.2/86-109, Rev. 10H and X3T9/89-042. According to the standards, a plurality of SCSI devices may be daisy-chained to one SCSI port. Moreover, through a SCSI controller, these devices may function without host supervision.
SCSI buses may exist in a "differential" configuration or a "single-ended" configuration. A single-ended configuration uses a single conductor in a cable of up to six meters in length for each signal line. A voltage level on the conductor determines an assertion or a deassertion of a signal line. Each single-ended SCSI connection has, for each signal line, a driver device with a single output terminal connected to that conductor, and a receiver device with a single input terminal connected to that conductor.
A differential configuration uses a pair of conductors in a cable of up to 25 meters in length for each of the SCSI signal lines. Each pair of conductors is connected to a positive line and a negative line of the bus. A voltage difference between those conductors (referenced to ground) determines the assertion or the deassertion of that line. Each differential SCSI bus connection has, for each signal line, a driver device with a pair of output terminals connected to the pair of conductors, and a receiver device with a pair of input terminals connected to the pair of conductors. Additionally, a ground terminal typically exists to provide a local ground plane, or a ground path, for the circuit.
Due to the large installed base of peripherals with single-ended SCSI interface, it is desirable to support single-ended devices as well as differential devices in a computer system. In such a system, the positive line of one differential driver device is connected to the input of a single-ended receiver device, while the negative line of the differential driver device is typically connected to a local ground plane. Although such an interconnection of single-ended devices to differential devices may be satisfactorily made with short cables, such interconnection may cause unexpected problems. These problems become particularly evident when a long cable is used to connect the devices together. The problems may include glitches in data/command lines of the SCSI bus.
The glitches are caused in part by a phenomenon known as cross-talk. Cross-talk relates to a mutual inductive coupling between electrical signals carried by the SCSI bus lines where transitions on one line causes false transitions on adjacent lines. This effect occurs when the voltage level of one electrical signal which is physically near another electrical signal switches. In such a case, a return current for the switching signal takes a least inductive path and if other nearby electrical signals share this path, a mutual inductive coupling effect may occur and may result in unreliable data transmissions, thus limiting data transmission speed and integrity.